A conventional pulse noise suppressing system of this type is shown in FIG. 1. In the system, an FM detection signal subjected to FM detection is applied through an input buffer (not shown) to a delay circuit 1 where it is delayed a predetermined period of time. The delayed FM detection signal is then applied to an analog switch 2. The FM detection signal is also applied to noise extracting means 3, namely, a HPF (High-Pass Filter) for extracting a noise component from the FM detection signal. The noise component passed through the HPF 3 is applied to noise amplifying means, namely, a noise amplifier 4, where it is amplified. The noise component thus amplified is rectified by a rectifier circuit 5, which outputs a noise detection signal whose level corresponds to the magnitude of the noise component. The noise detection signal is applied to a monostable multivibrator 6 which functions as a control signal generating circuit to generate an on-off control signal for controlling the on-off operation of the aforementioned analog switch 2.
When the level of the noise detection signal (which is applied to the monostable multivibrator 6), is higher than a predetermined value or level, the monostable multivibrator 6 outputs a pulse-like "off" control signal. The "off" control signal is applied to the control input terminal of the analog switch 2, to perform the "off" control of the analog switch 2. The output signal of the analog switch 2 is applied through a level hold circuit 7 to a stereo demodulation circuit (MPX). The level hold circuit 7 operates as follows: When the analog switch 2 is turned off, the level hold circuit 7 continuously holds the signal level provided before the analog switch 2 is turned off.
On the other hand, the rectifier circuit 5 also applies an automatic gain control signal (AGC signal) to the noise amplifier 4 for controlling the gain of the noise amplifier 4 according to the level of the noise component.
The cut-off frequency of the HPF 3 is set to about 100 KHz so that the HPF 3 transmits only a high frequency noise component included in the FM detection signal outputted by an FM detector circuit (not shown). The noise component extracted by the HPF 3 is amplifi.de by the noise amplifier 4 and then rectified by the rectifier circuit 5, so that the rectifier circuit 5 outputs a noise detection signal whose level corresponds to the noise level. When the noise is in the form of a pulse, the level of the noise detection signal is higher than the predetermined value or level. This noise detection signal triggers the monostable multivibrator 6, which outputs a pulse-like signal. The pulse-like signal is applied as the "off" control signal to the analog switch 2. When the noise level is high, the analog switch 2 is turned off to prevent the application of the noise to the stereo demodulation circuit (MPX).
The AGC signal applied to the noise amplifier 4 operates as follows. When the noise level increases, the AGC signal lowers the gain of the noise amplifier thereby preventing the analog switch 2 from being kept turned-off.
When the antenna input of a front end is low, the white noise inherent in an FM signal increases. If impulse noises are superposed on the white noise such that the impulses are above the threshold level as indicated at I in FIG. 2. then the noise components trigger the monostable multivibrator 6. As a result, the analog switch 2 is erroneously operated. If the analog switch 2 is operated in response to noises other than the pulse noise, then distortions or additional noises are produced, or the sensitivity is lowered.
In other words, when the receiving signal level is low and the noise level is not uniform, the conventional system shown in FIG. 1 does not operate properly.